The work proposed in this application is part of an on-going study of how the vertebrate nervous system generates and controls locomotor activity. The present study will seek to broaden our understanding of some of the ionic currents underlying single-cell behavior. In particular this study will record the ionic currents of single, isolated neurons from the spinal cord of a primitive vertebrate, the lamprey. The specific aims in studying these cells are to 1) investigate the mechanisms of presynaptic inhibition in sensory neurons and spinal axons, 2) determine whether the neurotransmitter serotonin alters calcium currents in branched neurons as a mechanism for reducing the late after-spike-hyperpolarization, 3) address the controversy regarding whether or not there are distinct glycine and gamma-aminobutyric acid receptors in the lamprey nervous system, and 4) determine whether N-methyl-D-aspartate (NMDA)-induced, tetrodotoxin-resistant membrane potential oscillations can occur in isolated neurons and determine the nature of their repolarizing current. The cells will be mechanically dissociated after a collagenase and pro- tease incubation. Different classes of lamprey spinal neurons will be identified morphologically or by pre-labelling with retrograde, fluorescent tracers In vivo. Ionic currents will be recorded in whole-cell voltage-clamp mode with patch electrodes, and individual ionic currents will be isolated by pharmacological means and by their characteristic voltage and kinetic properties. Drugs will be applied by bath perfusion to examine their effects on the isolated ionic currents. These studies will contribute to our basic understanding regarding the function of the spinal cord. Such understanding is important in the development of therapeutic measures to treat spinal cord injury and disease.